Ink-jet recording is known as a superior recording method that is not much affected by the types of recorded materials. Recording apparatuses, recording methods, recording materials and the like based upon ink-jet recording have been actively studied and developed. Hitherto, inks containing aqueous dyes composed mainly of water have been most commonly used for ink-jet recording inks. The inks are still most commonly used in present-day ink-jet recording because they have many such advantages as follows: coloring materials high in absorption coefficient and also high in color purity can be easily obtained to prepare them, the inks can be easily made multicolored so as to widen color exhibiting ranges, the inks' long-term storage stability and the inks' stability against heat can be secured, and most notably inks capable of reducing the occurrence of kogation can be produced.
However, since the dye inks are disadvantageous in terms of weatherability and water resistance, studies on pigment inks that contain pigments instead of aqueous dyes have been remarkable in recent years, and pigment inks are put on the market these days. Unfortunately though, pigment inks still remain more problematic than dye inks in color-developing ability and stability; and along with the improvements in techniques for increasing image quality, particularly in relation to printers for office automation, pigment inks have been required to exhibit printing quality, hues, color saturation, glossiness, storage stability and the like on plain paper that are comparable with those exhibited by dye inks.
Additionally, in general, magenta inks and cyan inks used particularly as pigment inks contain C. I. Pigment Red 122 and C. I. Pigment Blue 15:3 respectively, and thus pigment inks have color reproducing ranges that are different from those of dye inks. Also, in order to reduce hue errors, toning is often carried out, in which case the color saturation inevitably decreases, thereby causing a problem with printing quality.
Meanwhile, in order to change hues without depending upon toning, pigments themselves are improved as well. For instance, Patent Literature 1 proposes a cyan pigment having a hue in the same color gamut as that of a cyan dye, by employing a phthalocyanine pigment with a specific crystal structure; however, not all requirements are satisfied, as there are problems concerning costs, etc.
There are many other proposals including the proposal of Patent Literature 2, which discloses an ink set wherein a pigment is used for a coloring material for a black ink, whereas dyes are used for coloring materials for yellow, magenta and cyan inks. However, pigment-containing inks which have satisfactory printing properties on plain paper have not yet been obtained in reality.
Additionally, as an ink set including a black ink and color inks, Patent Literature 3 discloses an ink set including a black ink that contains a self-dispersible carbon black as a coloring material, and color inks that contain coloring materials, wherein the coloring material for the black ink and the coloring materials for the color inks have opposite polarities. Further, Patent Literature 4 discloses an ink set including inks in which colorant-containing resins are dispersed, wherein the inks have different ionicities. However, as to printed matter produced using these ink sets, although bleeding on boundaries between colors can be reduced, other printing properties on plain paper still remain unsatisfactory.
Meanwhile, as a low surface-tension aqueous pigment ink with stable ejection properties and improved wettability, for instance, Patent Literature 5 proposes an ink-jet printing ink containing a water-soluble organic solvent, a colorant, water and a perfluoroalkyl sulfonate. Additionally, as described in Patent Literature 6 to Patent Literature 8, there are some proposals of ink compositions for which fluorochemical surfactants are used. However, all these proposals present such a problem that in the case where pigments are used for colorants, the colorants are inferior in dispersion stability, fixation on recording media and color-developing ability. Also, in general, low surface-tension aqueous inks with improved wettability (including the inks of Patent Literature 5 to Patent Literature 8) present such a drawback that fluorochemical surfactants used in the inks cause the inks to foam greatly, and thus there are very serious effects on the ink feeding capability and the ejection stability of nozzles.
As for media, conventional ink-jet paper, particularly ink-jet glossy media, can be classified into swellable media and void-type media; these days, void-type media, which are superior in ink drying rate, are more popular. As these void-type media, media each incorporating an ink-absorbing layer that has voids through which ink is to be supplied onto a substrate, and also incorporating, if necessary, a porous glossy layer are most commonly used. As disclosed in Patent Literature 9 and Patent Literature 10, each of such void-type media can be obtained as follows: a coating solution in which silica or an alumina hydrate is dispersed is applied onto a substrate in one or more layers; and, if necessary, a glossy layer containing a large amount of colloidal silica is applied over the layer(s). Paper of this type is designed, with importance being placed on its compatibility with dye inks that are most commonly used at present, and the paper is already widely used as ink-jet paper, particularly as glossy paper. Use of the paper makes it possible to obtain very high definition output with high glossiness; on the other hand, since materials for the paper are very expensive and processes of producing the paper are complicated, the production costs of the paper are far higher than those of ordinary glossy coated paper for commercial printing. Thus, the use of the paper tends to be limited to cases where high-definition output such as photographic output is needed; and in reality the paper is difficult to use in the field of commercial printing where a great deal of output is required at low costs, for example in the production of handbills, catalogues, pamphlets, etc. These days, for the sake of higher image quality, the number of colors of inks used in printing tends to be increased, and the required ink absorbability tends to be increased as well. To increase the ink absorbability of media, it is reasonable to increase the thickness of ink receiving layers (coat layers); however, the thicker they are, the more expensive the materials therefor are, which leads to a rise in the unit prices of the media.
For pigments that form the ink-absorbing layers (receiving layers), it is necessary to use materials which are small in refractive index and have low concealing properties, in other words which are capable of keeping the transparency of the layers high and which absorb large amounts of oil (have large specific surface areas). Thus, in reality, there is no choice but to use large amounts of expensive low refractive-index, high oil-absorbing pigments such as silica or alumina hydrates, as opposed to inexpensive white pigments such as calcium carbonate and kaolin. Specifically, this is because if pigments having low transparency and high concealing properties are used for the ink-absorbing layers, coloring materials in inks that have soaked into the ink-absorbing layers are concealed by these pigments having high concealing properties, thereby causing a reduction in density. In fact, when paper which contains a pigment having high concealing properties is subjected to ink-jet printing with a dye ink, density is derived only from a coloring material present in the vicinity of a surface layer of the paper, no matter how increased the amount of ink to be applied is; thus, the density is low as a whole, and an image with little contrast is produced. Meanwhile, when a material which absorbs only a small amount of oil is used, ink absorption is insufficient, and thus beading easily arises.
Accordingly, these days, attempts to achieve a favorable balance between the refractive index and the whiteness are made by using fine organic particles having a small refractive index, as disclosed in Patent Literature 11; however, the fine organic particles, too, are high in production cost, and so it is still difficult to obtain inexpensive ink receiving paper which is compatible with dye inks.
As for the design concept for long-term storage stability of images produced, since dye molecules themselves are not highly resistant to ultraviolet rays or ozone, such a method is most commonly employed that a dye is made to soak into an ink receiving layer of a medium as deeply as possible so as to minimize the effects of the air and ultraviolet rays, and the dye is protected with an antioxidant or stabilizer previously added into an image receiving layer of the medium. Accordingly, by using a large amount of ink in which the concentration of a coloring material is relatively low, deep penetration of the ink is secured (ensured) and image storage stability is maintained. Consequently, the amount of ink necessary to output images thereby increases, which not only makes it difficult to miniaturize cartridges but also raises printing costs.
Judging from the above-mentioned points, in ink-jet recording, it is very difficult to provide inexpensive ink-jet paper and a printing method which are capable of high-definition output.
Meanwhile, in recent years, note has been taken of pigment inks for ink-jet recording. Since pigments are insoluble in water, pigment inks in which pigments are formed as fine particles and dispersed in solvents are generally used. As pigment inks for ink-jet recording, however, pigment inks in which pigments are dispersed in water are most commonly used in view of safety, etc. Generally, aqueous pigment inks easily cause flocculation or precipitation of pigment particles in comparison with dye inks; in order for the long-term storage stability of the aqueous pigment inks to be comparable with that of dye inks, various dispersion conditions and additives are necessary; also, dispersion stabilizers cause kogation; thus, the aqueous pigment inks are difficult to use with thermal heads, and also there is such a drawback that many of them contain coloring materials which are narrower in color exhibiting range than dyes. Nevertheless, the aqueous pigment inks have been attracting more and more attention for their printing quality, such as their capability of obtaining high black density, and for their storage stability and water resistance after recording. Ink-jet printers using the pigment inks are deemed able to approximate the texture of printed matter to that of printed matter produced by commercial printing because the coloring materials contained in the pigment inks are similar to those contained in ordinary commercial printing inks; however, when coated paper for commercial printing is actually printed with images or the like using conventional pigment inks, the pigment inks do not dry as quickly as they should, and thus the images or the like bleed, or pigments are not fixed at all after dried, for example; consequently, as in related art, the ink-jet printers are only suitable for printing onto media which have high ink-absorbing properties, such as plain paper and ink-jet paper. This is because the design concept concerning the formation of ink-jet images is no different from the concept in the case where dye inks are used; specifically, the pigments as coloring materials are merely viewed as dyes having high light resistance, and characteristics of the pigment inks are not considered at all.
Patent Literature 12 and Patent Literature 13 each disclose an image recording method employing an ink-jet recording method in which pigment inks are applied onto inexpensive general-purpose paper for commercial printing, not ink-jet paper. However, the image recording method presents the following problems: the inks are attached in large amounts to the inexpensive general-purpose paper for commercial printing, which necessitates spending a great deal of time in drying the inks; moreover, in this state where the inks are excessively attached to the printing paper surface, the inks having different colors become adjacent to one another before being absorbed into the printing paper, and thus bleeding between the different colors easily arises.
Patent Literature 14 discloses a method in which in order to produce a color proof using an ink-jet printer, a precoating solution having a function of flocculating a pigment is attached onto a recording medium before printing is performed by the ink-jet printer. However, this method is problematic in that the process of attaching the precoating solution makes operation complicated, and also in that when the precoating solution is attached onto the recording medium, the amount of water attached per unit area of the recording medium is large, which easily causes troubles in conveying the recording medium, such as curling and cockling.    [Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 2000-17207    [Patent Literature 2] JP-A No. 2000-239590    [Patent Literature 3] JP-A No. 10-140064    [Patent Literature 4] JP-A No. 2000-191972    [Patent Literature 5] JP-A No. 57-90070    [Patent Literature 6] JP-A No. 04-211478    [Patent Literature 7] JP-A No. 05-263029    [Patent Literature 8] JP-A No. 06-200200    [Patent Literature 9] JP-A No. 2005-212327    [Patent Literature 10] JP-A No. 11-078225    [Patent Literature 11] JP-A No. 2003-025717    [Patent Literature 12] JP-A No. 2002-67473    [Patent Literature 13] JP-A No. 2002-69346    [Patent Literature 14] JP-A No. 2003-211819